Photochromic materials are known and exhibit a change in light transmission or color in response to exposure to actinic radiation in the spectrum of sunlight. Removal of the incident radiation causes these materials to revert back to their original transmissive state.
Such photochromic materials have product applications for sunglasses, graphics, ophthalmic lenses, solar control window films, security and authenticity labels, and many others. The use of photochromic materials, however, has been very limited due to degradation of the photochromic property of the material from continued exposure to ultraviolet (UV) light radiation, particularly short wavelengths (<400 nanometers (nm)), and to infrared (IR) radiation (>780 nm). This degradation is known as “light fatigue” and is caused by chemical decomposition of the organic compounds which produce the photochromatic effect.
Photo-degradation of organic compounds involves two distinct chemical processes. One process is the absorption of UV radiation, or photolysis, and the accompanying formation of free radicals resulting from molecular bond breakage. The second process is auto-oxidation where the interaction of the free radical formed during photolysis with oxygen forms a peroxy radical, which attacks the molecular structure of the compound by hydrogen abstraction, generating another free radical. Further reaction of this free radical with oxygen leads to the formation of hydroperoxide, which generates more free radicals via photolysis.
This chemical decomposition is indicated by the gradual slowing down of the rate of bleach (inactive) to colored (activated) transformation. The total absence of reversible color change shows complete degradation or inactivation of the molecular entities responsible for the color formation. Many patents describe the use of light stabilizers in various combinations to improve the durability of photochromic articles (e.g., U.S. Pat. Nos. 4,440,672, 4,720,356, 5,000,878, 5,330,686, 5,391,327, 5,973,039, 6,083,427 and 6,262,155). However, they are insufficient for use in products involving long periods of exposure to sunlight. Therefore, there is a need for a photochromic material having improved durability and enhanced resistance to “light fatigue.”